It's from a video game. It's the evil giant dragon/turtle kind of creature (it has a spiked shell, but it also breathes fire) that has a lot of castles. It steals Princess Peach and Mario must rescue the princess, but he doesn't know what castle she's held at, so he has to find the right castle first as well. After the original Mario video game, there were also Mario Kart (a karting game) and many other video games with them. In Mariokart, Bowser is an adorably turtle, less of a bully.